Resonator method and system for distinguishing characteristics of surface features or contaminants

Abstract

A resonator method and system for distinguishing characteristics of surface features or contaminants provides improved inspection or surface feature detection capability in scanning optical systems. A resonator including a surface of interest in the resonant path is coupled to a detector that detects light leaving the resonator. Changes in the resonance peak positions and peak intensities are evaluated against known changes for standard scatters in order to determine the material characteristics of an artifact at the surface of interest that causes a resonance change. The lateral size of the artifact is determined by de-convolving a known illumination spot size with the changing resonance characteristics, and the standard scatterer data is selected in conformity with the determined artifact size. The differential analysis using resonance peak shifts corresponding to phase and amplitude information provides an identification algorithm that identifies at least one artifact/material type is identified from matching known behaviors of artifacts/materials.

1. An optical system, comprising:
 an illumination system for providing an illumination beam for illuminating a surface of interest through a first partially reflective surface; 
 at least one reflector including said first partially reflective surface for sustaining multiple internal reflections in a resonant cavity formed at least partially between said at least one reflector and said surface of interest, and wherein an extraction partially-reflective one of said at least one reflector permits light to exit said resonant cavity; 
 a detector for detecting an intensity of light leaving said cavity through said extraction reflector; 
 a scanning system for moving one of said illumination beam and said surface of interest to generate a relative movement between said surface of interest and said illumination beam; and 
 a processing system having an input coupled to said detector for identifying at least one material type of an artifact disposed at said surface of interest from a change in a resonance position and amplitude of said detected intensity caused by said relative movement across said artifact. 
 
   
   
     2. The optical system of  claim 1 , wherein said optical illumination system has a variable wavelength, and wherein said processor detects said change in resonance position by detecting resonance peaks occurring at distinct values of said variable wavelength and detects said change in amplitude by detecting said intensity of said light leaving said cavity at said resonance peaks. 
   
   
     3. The optical system of  claim 2  wherein said optical illumination system has a swept wavelength, and wherein said processor detects said resonance peaks and stores timing information associated with said resonance peaks, whereby a difference in illumination wavelength corresponding to said resonance peaks is determined, and wherein said processing system determines said change in resonance position from said timing information. 
   
   
     4. The optical system of  claim 1 , wherein said resonant cavity has a tunable cavity length, and wherein said processor detects said change in resonance position by detecting resonance peaks occurring at distinct values of said tunable cavity length and detects said change in amplitude by detecting said intensity of said light leaving said cavity at said resonance peaks. 
   
   
     5. The optical system of  claim 1 , wherein said processing system compares said change in resonance position and amplitude with predetermined values and determines a type of said artifact in conformity with a result of said comparison. 
   
   
     6. The optical system of  claim 1 , wherein said processing system further determines a size of said artifact by performing a deconvolution of a predetermined spot size of said multiple internal reflections incident upon said surface of interest from changes in said detected intensity caused by said artifact as said scanning system generates said relative motion across said artifact. 
   
   
     7. The optical system of  claim 6 , wherein said processing system further performs said identifying in conformity with predetermined resonance peak changes for a standard scatterer model selected in conformity with said determined size. 
   
   
     8. The optical system of  claim 7 , wherein said processing computes differences in an expected resonance peak shift and an expected resonance peak amplitude change for said selected scatterer model and a measured resonance peak shift and a measured resonance peak amplitude change, and wherein said identifying is performed in conformity with said computed differences. 
   
   
     9. The optical system of  claim 7 , wherein said standard scatterer model is a set of values for a calibrated scatterer and wherein said predetermined resonance peak changes are obtained by performing a calibration with said calibrated scatter. 
   
   
     10. The optical system of  claim 1 , further comprising:
 an optical coupler for receiving an output of said illumination system, and wherein said resonator is a measurement resonator coupled to a first output of said optical coupler; 
 a reference resonator coupled to a second output of said optical coupler for generating at least a second resonance within a path of said reference beam; and 
 at reference detector coupled to said reference resonator for measuring a reference intensity of light at an associated one of said at least one reference resonator, and wherein said processing system is further coupled to an output of said reference detector, and compensates for variations in said detected intensity from said cavity due to changes in effective length of said cavity and a wavelength of said illumination system in conformity with variations in said reference intensity. 
 
   
   
     11. The optical system of  claim 1 , wherein said illumination beam and said multiple internal reflections illuminate said surface of interest along an incidence angle away from normal in a resonant path of said cavity. 
   
   
     12. The optical system of  claim 11 , wherein said illumination system provides a polarized illumination beam, and wherein said detector further comprises:
 a polarizing beam splitter for splitting said light leaving said cavity into two output beams having different polarizations; and 
 two intensity detectors, each coupled to one of said output beams, and wherein said processing system computes polarizing characteristics of said artifact and said surface of interest at an intersection of said resonant path and said surface of interest in conformity with intensity outputs of said at least two detectors to determine changes in polarizing amplitude characteristics introduced by said artifact. 
 
   
   
     13. The optical system of  claim 12 , wherein said processing system further determines changes in said resonance position for each of said intensity outputs, and determines a polarizing phase relationship of said artifact from changes in resonance position as a wavelength of said illumination beam is changed. 
   
   
     14. The optical system of  claim 12 , wherein said processing system further determines changes in said resonance position for each of said intensity outputs, and determines a polarizing phase relationship of said artifact from changes in resonance position as an optical length of said cavity is changed. 
   
   
     15. A method of detecting optical characteristics of an artifact disposed at a surface of interest, said method comprising:
 repeatedly reflecting incident light in a cavity formed at least partially between at least one reflector including at least one partially reflective surface and a surface of interest; 
 detecting an intensity of light transmitted from said cavity through said at least one partially reflective surface; 
 moving one of said surface of interest and said at least incident light to generate a relative motion of said surface of interest with respect to said incident light; 
 changing an effective optical length of said cavity through a range of said effective optical length, at positions of said moving, whereby a range of said effective optical length can be studied at each of said positions; and 
 identifying at least one material type of said artifact from changes in a resonant effective optical length of said cavity and a magnitude of said intensity to values taken when said moving has located said incident light on said artifact from values taken when said moving has located said incident light on surface of interest away from said artifact. 
 
   
   
     16. The method of  claim 15 , wherein said changing changes a wavelength of said incident light. 
   
   
     17. The method of  claim 15 , wherein said changing changes an optical length of said cavity. 
   
   
     18. The method of  claim 15 , further comprising matching said changes in a resonant effective optical length of said cavity and said magnitude of said intensity with predetermined optical characteristics to determine a type of said artifact. 
   
   
     19. The method of  claim 15 , further comprising deconvolving a spot size of said incident light from a profile of said artifact as said moving is performed by processing said detected intensity to determine a width of said artifact. 
   
   
     20. The method of  claim 15 , wherein said incident light is polarized light and wherein said repeatedly reflecting incident light is performed at a non-normal angle at said surface of interest, wherein said method further comprises splitting said transmitted light into two beams representing different polarizations of said incident light, wherein said detecting detects intensities of both of said two beams, and wherein said computing further computes polarizing characteristics of said artifact from changes in a resonant effective optical length of said cavity and a magnitude of said intensity with respect to both of said beams. 
   
   
     21. An optical system, comprising:
 an illumination system for providing an illumination beam for illuminating a surface of interest through a first partially reflective surface; 
 at least one reflector including said first partially reflective surface for sustaining multiple internal reflections in a resonant cavity formed at least partially between said at least one reflector and said surface of interest, and wherein an extraction partially-reflective one of said at least one reflector permits light to exit said resonant cavity; and 
 means for identifying at least one material type of an artifact disposed on said surface of interest in conformity with a detected intensity of said light exiting said cavity.